[greenyoda]

See if you can talk to the people in your company who hired the contractor. They might at least be able to give you a high-level description of what the software is supposed to do and how it's supposed to work. They might even have specs that they prepared for the contractor or other design documentation.

If the contractor's software has no tests or is poorly written, it's going to be hard to add features to it or refactor it. You might want to read Working Effectively with Legacy Code[1] by Michael Feathers, which describes how you can get a handle on large bodies of legacy software.

[1] http://www.amazon.com/Working-Effectively-Legacy-Michael-Fea...